DirectAdmin MySQL root password is set within the installation process and written down to setup.txt. If you didn't change this password you can find the default one in the setup.txt, use the command below to see the contents of this file:

less /usr/local/directadmin/scripts/setup.txt

your password is stored in the line starting with mysql=

If you want to access mysql from ssh or console directly create a file in /root/ dir called .my.cnf:

touch /root/.my.cnf

and edit this file with the vi editor:

vi /root/.my.cnf

Add the lines below:

[client]user=rootpassword=XXXXXX

Fill the password with the pass you got from setup.txt, save the file with SHIFT+ZZ or :wq and exit. After this step just write mysql to enter MySQL console directly without password.